The present invention relates generally to reducing emissions from diesel engines and more specifically to electronically-controlled air injection to achieve simultaneous reduction of NOX and particulates emissions from a diesel engine.
Despite the numerous advantages and widespread usage of diesel engines, particulates (including soot and hydrocarbons) and oxides of nitrogen emissions from these engines are pollutants which engine manufacturers seek to minimize. Oxides of nitrogen, NO and NO2, commonly referred to as NOX, are formed by the reaction of nitrogen and oxygen from the combustion of the air-fuel mixture at high temperatures during the diesel combustion stroke. The high-temperature combustion of the diesel engine which is responsible for its high efficiency is also the cause of its NOx emissions. Particulates are conglomerates formed during combustion that may consist of large aromatic molecules with very high carbon-to-hydrogen ratios, inorganic species originating in the lube oil, sulfates from the oxidation of fuel sulfur, unburned fuel or lube oil, and products from the partial combustion of fuel or lube oil. It is difficult to control both particulates and NOx emissions simultaneously and cost-effectively without imposing significant penalties in fuel economy and/or power on diesel engines because of their inherent trade-off characteristics.
The diesel engine combustion process consists of an initial premixed stage in which chemical kinetics are rate controlling, followed by a diffusion stage in which mixing is rate controlling. Both NOX and particulate formation rates are generally high in the premixed stage because of higher energy release rates. The diffusion stage of the combustion process is generally associated with lower energy release rates than those of the premixed stage. The energy release at this stage is controlled by the mixing rate since, during this period, the characteristic time of combustion is much shorter than the characteristic time of fuel-air mixing. The mixing process is very important in determining the particulates oxidation rates. By enhancing turbulent mixing during the diffusion stage of the combustion cycle (where particulates oxidation is more pronounced) without disturbing the region of NOX formation, a reduction in visible smoke, hydrocarbons, and particulates in general can be achieved.
Previous analytical studies have shown that many factors will influence the effectiveness of late-cycle air injection. These include beginning of air injection, duration of air injection, air injection pressure, temperature, air composition, nozzle geometry (orifice diameter, number of holes, orifice aspect ratio), orifice orientation with respect to cylinder axis, location of the injector, beginning of fuel injection timing, fuel injection pressure, duration of fuel injection, fuel injection rate and shape, and composition of combustion air. Conceivably, the impact of late-cycle air injection would be significant if an air jet follows the fuel spray, or the combusting fuel spray plume. However, there is no practical (commercially available) system to perform direct combustion chamber air injection in diesel engines. Direct air injection through an additional injector might increase the system complexity and cost, and, hence, it is not preferable.
Accordingly, there is a clearly-felt need in the art for achieving simultaneous reduction of NOX and particulates in a diesel engine without reducing power output or fuel economy.
The present invention is an electronically-controlled late cycle injection of supplemental air to achieve simultaneous reduction of NOX and particulates emissions from a diesel engine without sacrificing its power output or fuel economy, wherein minimal modifications are required to the power assembly and fuel injection equipment, which modifications may be in the form of original engine manufacture or retrofit.
According to the present invention, a high-pressure jet of supplemental air is introduced into the cylinder (specifically, the combustion chamber thereof) late in the diesel cycle which serves to reduce particulate emissions over a broad range of engine operating conditions. When the supplemental air injection is coupled with NOX reducing methods, such as retarded fuel injection timing, multiple injections, or exhaust gas recirculation, simultaneous reduction of both NOX and particulate emissions is realized.
Controlled quantities of supplemental air, at relatively higher pressure than the combustion chamber combustion pressure, are introduced directly into the combustion chamber during the combustion (also referred to as the expansion) stroke or during the combustion and exhaust strokes of the diesel cycle. The mass of injected supplemental air is relatively small (about 2% to 10% of the total airflow) compared to the main intake airflow. The jet momentum of the supplemental air serves to augment turbulent mixing and also increases the partial pressure of oxygen in the gases surrounding the burning fuel droplets. These changes in mixing and chemical kinetics help to enhance the particulates (soot and hydrocarbon) oxidation and gas-phase hydrocarbon combustion reactions without adversely affecting NOX formation due to the supplemental air promoting a higher oxygen concentration and average lower temperature in the combustion zone. As a result of these enhanced oxidation reactions, visible smoke, unburned hydrocarbons, and total particulate emissions will be significantly reduced. The decreasing temperature, due to expansion and mixing of high-temperature gas with the supplemental air or cooler burned gas, freezes the oxides of nitrogen chemistry. This chemistry freezing effect occurs more rapidly in diesel engines and much less decomposition of the oxides of nitrogen occurs. The timing and delivery parameters for injection of diesel fuel, as well as the supplemental air, are electronically controlled so that NOX formation can still be reduced during the premixed and early diffusion stages of the diesel combustion cycle. The addition of supplemental air directly into the combustion chamber of a diesel engine has the potential to reduce both NOX and particulate emissions simultaneously, and lessens the trade-off between them.
These emissions reductions are achieved with the following major components:
a) a source of supplemental air in the form of a small portion of the engine""s intake air diverted from the intake manifold (filtered air at intake boost conditions);
b) a high-pressure compressor and/or a pump (piston, diaphragm or rotary type) to pressurize the supplemental air (for example, to pressures ranging up to 5,000 psi), wherein the pressure should generally be greater than the combustion chamber combustion pressure;
c) an accumulator or rail to hold the pressurized supplemental air and supply lines to the individual injectors;
d) an electronically-controlled solenoid valve to actuate the supply of the supplemental air into each injector, respectively;
e) an injector mounted on the cylinder head with one of more orifices to inject the supplemental air with a desired spray pattern directly into the respective combustion chamber; and
f) an electronic control system to control injection timing and delivery characteristics of the supplemental air during the combustion stroke or during the combustion and exhaust strokes of each cylinder, respectively, of a diesel cycle, wherein the beginning of supplemental air injection with respect to piston position in the cylinder, injection pressure, and duration of injection are all electronically controlled.
The electronically-controlled supplemental air injection system according to the present invention may be implemented in any of three different embodiments.
A first embodiment of an electronically-controlled supplemental air injection system includes a T-adapter, an air solenoid, a control unit, and a fuel injector. The T-adapter receives pressurized fuel from a fuel pump through a fuel check valve and pressurized air from a source of compressed air through an air solenoid and then an air check valve. The pressurized fuel and supplemental air are output from the T-adapter through the fuel injector, wherein the air solenoid controls the flow of supplemental air to the fuel injector. The air check valve prevents fuel from entering the air line and the fuel check valve prevents air from entering the fuel line. A computer control device dynamically controls the opening and closing of the air solenoid according to timing appropriate to a predetermined specific cycle of a diesel engine and the operating conditions of the engine.
A second embodiment of an electronically-controlled supplemental air injection system includes a modified fuel injector, an air solenoid, and a control unit. The fuel injector is modified by forming an air passage through an outside perimeter to a sac thereof. The sac is located below a needle of the fuel injector. Compressed supplemental air is supplied through an air solenoid to an input of the air passage on the outside perimeter of the fuel injector. The supplemental air is supplied to the sac when the control unit opens the air solenoid. The supplemental air flows from the sac through an orifice in a tip of the fuel injector. A computer control device dynamically controls the opening and closing of the air solenoid according to timing appropriate to a predetermined specific cycle of a diesel engine and the operating conditions of the engine.
A third embodiment of an electronically-controlled supplemental air injection system includes a cylinder head with at least one air passage, at least one cylinder passage valve, an air solenoid, and a control unit. The cylinder head will not have to be modified if it already contains cylinder relief passages, as for example for the purpose of testing and safety procedures. A cylinder head not having cylinder relief passages is modified by adding at least one air passage per cylinder. The cylinder relief valves are replaced with cylinder passage valves or added to the air passages in the modified cylinder head. Compressed supplemental air is supplied through the air solenoid to each cylinder air passage. The supplemental air passes into a particular combustion chamber when the control unit opens the air solenoid and the combustion chamber""s respective cylinder passage valve. A computer control device dynamically controls the opening and closing of the air solenoid according to timing appropriate to a predetermined specific cycle of a diesel engine and the operating conditions of the engine.
With regard to the aforementioned timing, the supplemental air is injected into a combustion chamber during, preferably, one of two specific time periods during the diesel cycle. In a first specific time period example, supplemental air is injected into the combustion chamber when the piston is at top dead center (TDC) at the start of the combustion stroke and continues until the exhaust valve is opened. In a second specific time period example, supplemental air is injected into the combustion chamber when the piston is at TDC at the start of the combustion stroke and continues until the piston is at TDC at the end of the exhaust stroke.
Accordingly, it is an object of the present invention to provide electronically-controlled supplemental air injection to achieve simultaneous reduction of NOX and particulates emissions from a diesel engine without reducing power output or fuel economy thereof.
This and additional objects, features and advantages of the present invention will become clearer from the following specification of a preferred embodiment.